In geophysical prospecting, seismic operations are frequently used to generate, collect, and analyze information about subsurface formations. Such seismic operations are typically performed by initiating seismic disturbances at a point near or at the surface of the earth so that seismic waves are generated downward into the earth at that point. These seismic waves, or acoustical signals, travel downward into the earth until they encounter discontinuities in the earth's structure in the form of varying subsurface strata formations. Such discontinuities reflect at least part of the acoustical signals back toward the earth surface. In oil and gas exploration operations, these reflected acoustical signals are recorded and studied to help locate and analyze various subsurface formations for potential oil and gas production.
In oil and gas operations, seismic energy sources, such as dynamite or blasting cord, are frequently used to generate the acoustical signals. Also, large truckmounted seismic sources such as vibrators or thumpers are used to generate the acoustical signals.
After the acoustical signals have been generated and then reflected by the subsurface formations, the reflected signals are measured and recorded at various locations on the ground surface by sensitive geophones or other seismic receivers for interpretation. These recorded signals are then studied to determine the likelihood that a given subsurface strata contains producible hydrocarbons.
One variation of the typical seismic exploration method mentioned above is called Vertical Seismic Profiling ("VSP"). VSP is known to be valuable in structural and stratigraphic interpretation of subsurface formations and geological prospecting for oil and gas. In VSP, a geophone or other type of acoustic detector is lowered into a wellbore. Acoustical signals are then generated at various ground surface locations offset from the wellbore. Recordings are made through the geophone at various levels in the wellbore. This differs from usual seismic operations which have both the seismic source for generating the acoustical signals and the receivers at or near the ground surface.
In VSP, the acoustical signals travel from the signal source through the near ground surface only once on their way to the geophone in the wellbore. This results in less attenuation of high frequency waves than occurs for typical surface seismic operations where the signals must travel through the near ground surface twice. These higher frequencies give VSP better resolution than surface seismic methods.
A disadvantage of VSP is that numerous offset energy source locations, some at large distances from the wellbore, are required to obtain the amount of seismic information necessary to properly study a given subsurface formation. Placement of these offset energy sources is time consuming and expensive. Often the placement of the seismic energy sources, such as dynamite, blasting cord, or large vibrators or thumpers, at a desired location is difficult. Seismic sources must be kept some distances from buildings, dwellings, roads, and other structures that would be affected by blasting or the use of dynamite. Also roads to isolated exploration areas may not allow for transporting large pieces of seismic source equipment to required locations.
In order to obtain the benefits of VSP in areas where using a seismic source to create acoustical signals from a surface location may be difficult, a modified VSP method, referred to as reversed VSP, is used. In reversed VSP, a seismic source is placed in the wellbore and geophones or other type of acoustical detectors are laid out on the surrounding ground surface. The surface receivers can be located in positions that would not permit the use of dynamite or are inaccessible to seismic sources such as large vibrators. In addition to being useful in places conventional VSP cannot be used, reversed VSP is capable of obtaining higher quality data than conventional VSP. In reversed VSP operations, receivers can be buried in complicated arrays which improve the frequency content of the reflected signals and reduce noise in the reflected signals. Accordingly, higher frequency and more consistent data can be recorded with reversed VSP than with conventional VSP. The most significant advantage of reversed VSP is that a single downhole seismic source, if used with a large number of geophones at the ground surface, can generate data equivalent to many standard VSP operations with various offsets.
A seismic operation similar to reversed VSP is cross-hole seismology. In cross-hole seismology, a seismic source is lowered into one wellbore and a geophone is lowered into a second wellbore. The seismic source creates acoustical signals that travel from the first wellbore to the second wellbore where the signals are measured and recorded. Cross-hole seismology does not require the laying out of surface geophones as is required in reversed VSP. Because the acoustical signals do not have to travel through the near ground surface, seismic data is produced having high resolution and a high signal-to-noise ratio. Cross-hole seismology is most generally used in a producing field, where existing wellbores may be used to provide additional information about previously discovered reservoirs.
There are various downhole energy sources available for use in reversed VSP and cross-hole seismology. Early methods for generating acoustical signals included the use of large wrappings of explosive blasting cord, sidewall coring guns, and perforating guns. Although these methods could provide an energy source of acceptable intensity for the generation of acoustical signals, blasting cord allowed only a single explosion for each downhole trip and sidewall coring and perforating guns were very damaging to the casing or wellbore. Currently, individual explosive charges or series of explosive charges without the damaging effects of the above mentioned guns are frequently used. These charges are electrically detonated from the ground surface by a seismic crew through a standard seven conductor wireline cable. The use of a standard wireline cable, however, limits the number of individual explosions available to be fired on a single downhole trip with a downhole seismic source.
Another downhole seismic source currently used is an air gun. An air gun arrangement uses a firing control line and a high pressure air hose to produce downhole acoustical signals. An advantage an air gun has is that it may be moved up and down the wellbore and repeatedly fired at various positions on a single downhole trip. However, an air gun has mechanical limitations and use restrictions that can make its operation and handling difficult. The firing control line and high pressure air hose are very bulky and can be difficult to operate in a deep wellbore. Also, a downhole air gun usually produces less acoustical energy than a 10 gram explosive charge of a standard pentaerythritol tetranitrate (PETN) explosive. Because of this relatively weak energy source level, air guns are usually used only for cross-hole seismology and not for reversed VSP.
Another disadvantage of using an air gun is that air guns produce more tube-wave energy in the wellbore than do downhole explosives. Existence of such tube waves (referred to as "noise") complicates data processing and interpretation of the recorded data. Additionally, air bubbles are produced during operation of an air gun which change the acoustical properties of the mud column, which in turn affects the tube wave velocity and further complicates signal processing. Finally, an air gun's performance may be adversely affected by large hydrostatic pressures such as when the gun is operated at significant depths.
The downhole energy source that generates the most desirable acoustical signals is an explosive charge, such is obtained in firing a sidewall coring gun or perforating gun. Explosive shot arrangements, without the damaging effects of coring guns or perforating guns, are commercially available. The firing of these explosive shot arrangements can be controlled at the ground surface through a standard seven conductor wireline cable. A limitation in using standard surface firing control equipment with a standard seven conductor wireline cable is that only up to six individual or group shots can be fired before a downhole firing arrangement must be removed from the wellbore and reloaded. Since in typical reversed VSP and cross-hole seismology operations, the firing of hundreds or even thousands of shots might be required to generate the necessary amount of seismic information, a downhole firing apparatus using a standard surface firing control arrangement would require many downhole trips. Such numerous trips are time consuming, expensive, and prevent quick gathering of large amounts of data.
A downhole seismic source is required that is capable of firing a large number of independent explosive shots on a single downhole trip using a standard seven conductor wireline cable. The present invention provides this capability through the use of a downhole firing circuit to select and fire numerous explosive shots in a single downhole trip.